This invention relates to a protective system for an inverter circuit employed in driving an AC motor.
DC motors have long been adopted for such uses as spindle motors in machine tools and crane motors, but in recent years this field has seen increasing application of AC motors, particularly of the induction type, owing to the fact that they can now be controlled in speed over a wide range in the same manner as the DC motors which they are gradually replacing.
The known AC motor speed control systems generally include an inverter circuit for converting a direct current into an alternating current, the direct current being supplied by a battery or obtained by rectifying a commercial three-phase AC power source, as well as an inverter drive circuit for varying the frequency of the alternating current delivered by the inverter circuit. This alternating current is then applied to an AC motor which can be rotated at the desired speed by varying the frequency of the alternating current at will.
In the inverter circuit the conversion of direct current to alternating current is accomplished by repeatedly switching thyristors on and off if the inverter circuit is of the thyristor type. However, one disadvantage of the inverter circuit that relies on thyristors is that it must be separately provided with a circuit to turn the thyristors off since the thyristors do not have the ability to perform this function themselves even if they can turn themselves on.
In recent years, high output power transistors capable of controlling motors in the several tens of kilowatt class have been developed, and this has in turn led to the development of AC motor speed control systems which employ inverter circuits composed of such high output power transistors. These inverter circuits are much simpler than those employing the thyristors since each power transistor is controlled merely by the application of a base signal which renders the transistor conductive or non-conductive. One problem encountered in this system is that the transistors are likely to break down if an electric current in excess of a fixed value which is decided by the transistors should flow through them, even momentarily. Moreover, if the current is lower in level than this fixed value but an overcurrent which exceeds the rated value should flow through the inverter circuit for a long period of time, the transistors will still be damaged. To prevent such damage it has been conventional practice to provide the speed control system with a protective circuit adapted to immediately terminate the function of the inverter circuit as soon as it is subjected to an overcurrent.
The disadvantage with this arrangement is that it diminishes working efficiency since a running machine tool or belt conveyor will be shut down whenever an operator commits a minor error that causes an overload. In other words, employing a transistor-type inverter circuit to control the AC motors in such machinery renders the machinery susceptible to shut-down every time an overload occurs.